1. Field of the Disclosure
The present invention relates generally to power converters, and more specifically to power converters utilized with dimmer circuits.
2. Background
Electronic devices use power to operate. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter, a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. The switched mode power converter control circuit usually provides output regulation by sensing one or more inputs representative of one or more output quantities and controlling the output in a closed loop. In operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency, or varying the number of pulses per unit time of the switch in a switched mode power converter.
In one type of dimming for lighting applications, a triac dimmer circuit typically disconnects the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is known as phase dimming because it is often convenient to designate the position of the triac dimmer circuit and the resultant amount of missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the triac dimmer circuit disconnects the ac input. As such, removal of half the ac input voltage in a half line cycle by the triac dimmer circuit corresponds to a phase angle of 90 degrees. In another example, removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees.
On the other hand, the conduction angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle that the triac dimmer circuit does not disconnect the ac input voltage from the power converter. Or in other words, the conduction angle is a measure of how many degrees of each half line cycle in which the triac dimmer circuit is conducting. In one example, the removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees but a conduction angle of 135 degrees.
Although phase angle dimming works well with incandescent lamps that receive the altered ac input voltage directly, it typically creates problems for light emitting diode (LED) lamps. Most LEDs and LED modules are best driven by a regulated current which a regulated power converter can provide from an ac power line. Triac dimmer circuits typically don't work well with conventional regulated power converter controllers. Regulated power supplies are typically designed to ignore distortions of the ac input voltage. Their purpose is to deliver a constant regulated output until a low input RMS voltage causes them to shut off completely. As such, conventional regulated power supplies would not dim the LED lamp. Unless a power converter for an LED lamp is specially designed to recognize and respond to the voltage from a triac dimmer circuit in a desirable way, a triac dimmer is likely to produce unacceptable results such as flickering or shimmering of the LED lamp with large conduction angles and flashing of the LED lamp at low conduction angles.
Thus, a power converter may include an improved conventional power converter controller that is designed to respond to a triac dimmer circuit by directly sensing the average value of the dimmer circuit output (in other words, the average value of the ac input voltage after the triac dimmer circuit has processed the ac input voltage) to determine the amount of dimming requested. In general, a smaller average value of the dimmer circuit output would correspond to a removal of a greater portion of the ac input voltage and thus a larger phase angle. As such, the improved conventional power converter controller utilizes this relationship to indirectly determine the phase angle and alter the quantity to which the output of the power converter is regulated. However, by indirectly measuring the phase angle in this manner, the amount of dimming detected (and hence the quantity to which the output of the power converter is regulated) is subject to variances of the ac input voltage. In other words, the accuracy of the phase angle measured through the average value of the dimmer circuit output is dependent on variances of the ac input voltage.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.